A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).
Transistors including oxide semiconductors have on-state characteristics (e.g., on-state current) superior to those of transistors including amorphous silicon.
In addition, as for such an oxide semiconductor used in a transistor, there is also description as follows: an oxide semiconductor is insensitive to impurities, there is no problem when a considerable amount of metal impurities are contained in a film, and soda-lime glass which contains a large amount of alkali metals such as sodium and is inexpensive can also be used (see Non-Patent Document 1).